1. Field of the Invention
This invention relates to a thermal recording head, and more particularly to a thermal recording head suitable for recording halftone images by use of a thermal transfer arrangement.
2. Description of the Prior Art
Thermal transfer recording, ink-jet recording and electrophotographic recording are conventional techniques to achieve nonimpact printing for recording images on plain paper. Of these recording techniques, thermal transfer recording has the advantages of maintenance-free apparatus, easy operation, simplified configuration, and colored recording. Thus, the thermal transfer recording technique is widely utilized for printers of personal word processors, graphic printers and the like.
FIG. 6 shows a conventional thermal transfer printer. In FIG. 6, a platen roller 102 is disposed on a thermal recording head 101. Recording paper 103 and an ink ribbon 104 are sandwiched between the head 101 and roller 102. The paper 103 and ink ribbon 104 move together between the platen 102 and the thermal head 101 in the direction of the arrow as the platen roller 102 rotates. Thus, the paper 103 and ink ribbon 104 move at a specified speed in the arrow-marked direction.
FIG. 7 is an enlarged view in detail of a portion of the configuration of thermal recording head 101. In FIG. 7, a large number of very thin heating resistors 101a (4 to 16 dots/mm, for example) are respectively connected between a plurality of pairs of electrodes 101b and 101c. These resistors 101a are disposed in a single row, each isolated by insulating elements 101f. A large number of driver-transistors 101e are respectively connected to the heating resistors 101a through corresponding electrodes 101c. These transistors 101e individually perform ON-OFF control with respect to power supplied from a power source 101d. Means not shown, such as a microprocessor plus a driver circuit, are conventionally used to energize transistors 101e. Specifically, only specific resistors 101a corresponding to images to be recorded are energized to generate heat. As shown in FIG. 6, ink particles of the ink ribbon 104, which are adjacent the selectively energized heating resistors 101a, are melted to adhere to the recording paper 103 as the ink ribbon 104 and paper 103 move between the platen 102 and the printing head 101. Thus, ink particles 105 corresponding to images to be recorded are transferred to the paper 103. The other ink particles 104a, which are not transferred, remain on the ink ribbon 104.
This thermal printer performs two-valued recording, i.e., whether or not ink particles 104a adhere to the recording paper 103. Thus, in order to record halftone images, some particular arrangements are required. For example, a two-valued dither method is usually used. In this method, the dot density within a matrix constituted by (M.times.N) dots is area modulated to represent (M.times.N+1) tones corresponding to halftone images.
FIG. 8 shows an example of a four-dot (2.times.2) matrix for representing a five-tone level according to such a dither method. However, in actual cases, a 4.times.4-dot matrix through a 8.times.8-dot matrix are usually used.
However, the two-valued dither method is based on an area modulation to achieve a multi-tone recording. Thus, when the number of tones is increased, the size of the matrix for a given area becomes larger. As a result, the resolution of images is lowered. However, when the size of the matrix is reduced to enhance the resolution, the number of tones is reduced. Namely, to achieve multi-tone recording and high-resolution recording at the same time is difficult.
To solve this problem, the shape of the heating element within a thermal recording head has been improved in the prior art. Thus, only one dot can represent halftone images in an analog fashion. Here, "analog fashion" is understood in the art to mean that a heating element is energized in proportion to the turn-ON periods of the driver-transistor. The turn-On periods are controlled in accordance with the pulse widths of input signals to the driver-transistor. This method was disclosed in Japanese Patent Publications No. 60-78768 and No. 61-241163.
FIG. 9 shows a heating element 200 within a thermal recording head which is disclosed in Japanese Patent Publication No. 60-78768. The heating element 200 is connected between a pair of electrodes 201 and 202. The center of heating element 200 is narrowed to form a double concave-lens shape. As a result, heat generated by the heating element 200 becomes highest at the center where the electric current density is highest. The heat becomes lower towards either electrode. A thermal recording head that incorporates the heating element 200 has characteristics between recording density and recording energy as shown in FIG. 10. Recording energy is proportional to the current through element 200. FIG. 11 shows recorded dot-shapes "a" through "e" printed on the paper which correspond respectively to points "a" through "e" in the graph of FIG. 10.
The areas of recorded dot-shapes "a" through "e" of FIG. 11 are all the same as the area of ink melted by the heating element 200. As shown in FIG. 11, such area expands from a dot-shape at the heating center in a concentric fashion. Thus, when the diameter of the dots becomes greater, the heating element 200 conveys more heat out to the board to which the thermal recording head is attached, i.e., to the side opposite the recording surface. As a result, the recording density does not increase in proportion to the recording energy as shown in FIG. 10. The corners of the pixel remain blank as shown in "e" of FIG. 11. Consequently, the variable range of recording density narrows. Therefore, to extend the range of recording density, the temperature at the heating center must be raised to an extremely high level. However, if the thermal recording head is operated under such a severe condition, its service life is shortened significantly.
Moreover, when an image recording is performed in an analog fashion by use of one-dot unit per pixel, the respective dots within the adjoining pixels appear to couple with each other to the eye of the observer as shown in FIG. 12. This can occur when both adjoining pixels have the dot areas as shown in "c" of FIG. 11. The variations of the adjoining dot areas caused by such unavoidable coupling provide image-roughness to the naked eye. This phenomenon deteriorates the image quality.
On the other hand, FIG. 13 shows a different prior art heating element 300 of a thermal recording head which is disclosed in Japanese Patent Publication No. 61-241163. The heating element 300 is formed in a lattice configuration so that four narrowed sections form the heating portions of the element 300. This heating element 300 is connected between a pair of electrodes 301 and 302.
Because the heating portions of the heating element 300 are dispersed, the variable range of recording density can be expanded. However, the image resolution is lowered. Moreover, the quality of recorded image deteriorates because of image-roughness which is similar to the case of the heating element 200.